Crustal studies of the Koyna region using explosion data from deep seismic soundings

Tectonophysics,

110 (1984) 61-72

Elsevier Science Publishers

61

B.V., Amsterdam

- Printed

in The Netherlands

CRUSTAL STUDIES OF THE KOYNA REGION USING EXPLOSION FROM DEEP SEISMIC SOUNDINGS

H.N. SRIVASTAVA, Indra Meteorological

(Received

December

R.K. VERMA. Deparrment,

G.S. VERMA

DATA

and H.M. CHAUDHURY

New Delhi (India)

22, 1983; accepted

June 20. 1984)

ABSTRACT

Srivastava,

H.N., Verma,

region using explosion

R.K.. Verma,

G.S. and Chaudhury,

This study is based on the seismic data collected and 1978 Deep Seismic Sounding

Refraction velocities

studies

of the records

of the phases respectively

respectively.

reveal a two-layered giving the average

those

of Ss, S* and

of the Koyna

carried

out during

the 1976

region. These shots were exploded

Institute

along the Guhagar-Chorochi

and

In some cases reflections signal

17 km of granite

as 5.82+0.01, 4.09+0.07

6.61 kO.05 and

and 8.23*0.05

4.60+0.08

km/set

rock has been found to be lower in comparison

with

India. were recorded

thickness

durations

heterogeneity

crust. The top layer consists

depth of the Moho as 36 km in the region. The

S, as 3.41+0.00,

The shear wave velocity in the basement

These reveal a crustal Coda

Research

Ps, P* and P, have been computed

and

other regions of the peninsular

lateral

Geophysical

in the Koyna

studies

110: 61-72.

profiles.

and the second layer 19 km of basalt, km/set

as a result of explosions

(DSS) field operations

from twelve shot points by the National Kelsi-Loni

H.M., 1984. Crustal Tectonophysics,

data from deep seismic soundings.

from

both from the Moho as well as from the intermediate

of 39 km with 19 km of granitic DSS explosions

recorded

in the crust on either side of Karad

and 20 km of basaltic

by microearthquake in an east-west

layer.

layers.

seismographs

indicate

a

direction.

INTRODUCTION

The Deccan plateau was considered to be a stable shield until the occurrence of the Koyna earthquake of December 10, 1967, with a magnitude of 6.5. This necessitated a re-evaluation of the seismicity of the Deccan region, particularly of the area surrounding the Koyna Dam. One of the recommendations in this connection was to carry out Deep Seismic Sounding (DSS) in the region. This work was completed in two field seasons, called Koyna Phase I and Koyna Phase II, by the National Geophysical Research Institute, Hyderabad. In the first field season a total distance of 220 km was surveyed along an east-west profile (Guhagar-Chorochi) across the western ghats, while in the second field season a distance of 200 km across 0040-1951/84/$03.00

0 1984 Elsevier Science Publishers

B.V.

62

another profile (KelsiiLoni) was surveyed (Fig. 1). The charge of the shots exploded varied from 50 to 1500 kg. The India Meteorological Department participated in both

these

peninsula

field seasons investigated

and longitudes data, specially

for collection

in this study is roughly bounded

73“.0E-75 during

of seismological

‘.2E

the Koyna

around

the Koyna

data.

The region

by latitudes

dam.

of the

16.7 ’ N --18.2’ N

In view of the paucity

Phase II. the results obtained

during

of

both the field

seasons have been combined. Kaila et al. (1981) have prepared discontinuity for the Koyna region

a detailed structural contour map of the Moho using the same D.S.S. shots. They have found

that the longitudinal wave velocity in the Deccan Traps along the profile varies from 4.8 to 5.0 km/set and in the crystalline basement from 6.0 to 6.15 km/set. While the DSS technique used by them gives a finer crustal structure, the wide range of seismic wave velocities cannot be adopted for the determination of epicentral parameters of earthquakes which need specific values of wave velocities averaged over the region. A direct approach was, therefore, adopted by the India Meteorological Department. A network of seismological observatories had to be opened along the profiles to give directly the average wave velocities and structure in the region and the results have been discussed in this paper. The data thus obtained could be used for improving the location of earthquakes in the Koyna region where seismic activity is still continuing.

GEOLOGICAL

SET UP OF THE REGION

Peninsular India, consisting of Precambrian rocks of very ancient origin, probably greater than 3000 m.y.. represents a stable block of the earth’s crust which has remained totally unaffected by the mountain building activity in the close of the Precambrian era (Krishnan, 1968). It is made up of the Archean gneisses and schists penetrated by igneous Dharwarian, Cuddapah

rocks and Precambrian sediments: the latter include the and Vindhyan formations. The earlier rocks are mostly

igneous in origin, while the latter comprise of both the igneous and sedimentary rocks. The entire area along the DSS profiles in the Koyna region are covered by the Deccan

Trap, consisting

of massive basalts,

vesicular

and amygdoloidal

basal&

tuff

breccia and red bole. The lava flows exhibit a westerly dip of 3’-4’ or more in the low region of Konkan while in the high plateau, a very gentle easterly dip of 1o has been reported (Das and Ray, 1973). The upper lavas are of upper Cretaceous or Paleocene age and thickness of these flows varies from one meter to over 40 m. Based on geomorphological evidence, Pascoe (1964) has suggested that the great scarp on the Western Ghat is due to a boundary fault with down throw on the western side. Auden (1975) opines that the faults located near the tail race tunnel exists of the Koyna project form part of the regional fracture pattern. A number of hot springs also occur in the region under study parallel to the west

63

coast in a 320 km long belt. According fault zone within OBSERVATIONAL

NETWORK

Five temporary Athini

to Gubin

(1969), the springs

arise along

a

the basement. AND DATA

seismological

and Tasgaon

during

Goregaon, Mahabaleswar, phase II field operations

observatories

the Koyna

were set up at Koyna,

phase I field operations

Karad,

Saverde,

in 1976 and five at

Phaltan, Malsiras and Poona in 1978 during the Koyna (Fig. 1). At all these stations, high magnification electro-

magnetic seismographs were installed for recording the explosions. Details of the stations and instrumental constants are given in Table 1. The recording was done on a fast run recorder (1 cm = 1 set). For maintaining time accuracy, all the stations were provided with crystal clocks with facility to impinge time signals on the record from a radio receiver. Special arrangements were made for impinging shot timings on the records. In addition to the electromagnetic seismographs, one highly micro-earthquake instrument (Sprengnether MEQ-800) was also installed with a frequency band of 5-10 Hz and a gain of 90 dB. A careful analysis of the records of all the events indicated the presence crustal phases P, and S, as the first arrivals. The refracted phases P* sub-crustal phase P, and the corresponding S phases were also recorded

7YO

45”O

15”o’ 1

f

30*0

30-0, I

0

RSS PROFILLE SHOT POlNT

0

SEISMOLOGICAL STATIONS

18’0

45OO’ 1

74”O’ 1

15*0* t

30°0’ ,

190’

3O”O’

45*0* 1

75OO’ I

Cl POONA

ABALASWAR

30’0

1 5’0

L

73 0

1500

30’0’

74’0’

Fig. 1. D.S.S. profiles and location of shot points and observatories

4YO’

75*0’

of direct and the at a few

45-o’

45*0

17%

sensitive at Karad

E

64

TABLE

1

Coordinates Stn.

of observatories

Observatory

and instrument

Symbol

No.

constants

Coordinates lat. a N

Magnrfica-

Instrumenta long. o E

seismometer

galvano-

free period

meter free

(secj

tion

pemd (sec.)

1

Athni

ATH

16 *43’34”

75 o 03’48”

1.5

0.5

SO K

2

Savarde

SVD

17 o 24’02”

73 o 32’30”

1.5

0.5

50 K

3

Karad

KRD

17~18’19”

74010’59”

1.5

0.5

75 K

4

Koyna

KOY

17’23’51”

73O45’00”

1.5

0.5

100 K

5

Tasgaon

TAS

17’02’11”

74=‘36’20”

1.5

0.5

6

Goregaon

GOR

18OO9’8.2”

73O17’23”

1.5

0.5

50 K

7

Mahabaleswar

MBL

17°55’13.8”

730 39’6”

1.5

0.5

50 K

8

Malsiras

MLS

17~51’40.8”

74O54’28.2”

1.5

0.5

75 K

9

Phaltan

PLT

17’59’19.2”

74O25’43”

1.5

0.5

100 K

Poona

PO0

1S019’12”

73O30’36”

1.o

0.75

SO K

10

(wwssn)

Distance

Fig. 2. Travel-time

(km)

curves for direct and refracted

phases.

SU K

65

stations

(Fig. 2). Reflections

recognised Muller

with confidence and Landisman

explosions

(1966) have identified

which has been attributed

phase was not discernable absence

from the intermediate

REFRACTION

in separate

a P, phase

to the presence

from the records

of a low velocity

layer and Moho could

as they were all recorded

layer as confirmed

from records

of a low velocity

of Koyna

also be

groups. of near

layer. Such a

region which may imply

the

from the DSS results.

DATA

Velocity structure Distances of the stations from shot points were computed using an IBM 360/44 Computer and the transit time of different phases (T) against the distance (A) was reduced with reference to an arbitrarily chosen mean velocity for P and S waves. Thus the data were fitted to straight lines using the method of least squares (Fig. 3). The values of the intercepts,

slopes, and the limits of errors are given in Table 2.

Crustal thickness The small intercept of Pa in the T-A curve suggested that the overlying low-velocity sedimentary layer is either absent or has a very small thickness.

TABLE

2

Standard

deviations

and standard

errors in wave velocities

and intercept

times

Stn. No.

Phase

V

OV

S.E.( V)

oa

S.E.( (I)

1

P,

5.82

0.10

0.01

0.08

0.97

0.13

2

P*

6.61

0.20

0.05

2.81

1.78

0.49

3

P”

8.23

0.20

0.05

7.62

1.38

0.39

4

Ss

3.41

0.04

0.00

0.84

0.15

a

-0.15

5

S*

4.09

0.19

0.07

5.74

3.92

1.60

6

S”

4.60

0.23

0.08

11.14

4.04

1.42

TABLE

3

Wave velocities Stn. No.

and thickness Rock layer

of crustal

layers from refraction

data

Velocity (km/set)

Thickness

P-wave

S-wave

P-wave

(km) S-wave

mean

1

granite

5.82

3.41

17.3

17.8

17.5

2 3

basalt upper mantle

6.61 8.23

4.09 4.60

19.0 _

18.5 _

18.8 _

4

depth of Moho

36.3

36.3

36.3

9.0

TO-

O

60.

c .,

0

40

80

120

Cistance

a

160

200

(km)

120.

l

100r

EDs

_

0

40

Distance

b

Fig. 3. Reduced

I 120

80

travel-rime

(T-A/‘\‘)

-1-_

1_L 160

_ 200

1 km)

curve.\.

Reducing

velwltv

being

(a) X.0 and (h) 4.6 km/xc.

Assuming

a two-layered

were calculated

crustal

for different

model the wave velocities

phases using the following

and thickness

of the layers

formula:

where 1; and f,_ are the intercepts made by the refracted waves from the Conrad and Moho boundaries respectively and H, and Hz the corresponding depths. The average thickness of the crust as computed from P and S phases separately was found to be 36 km, with 17 km of granitic layer and 19 km of basaltic layer (Table 3). REFLECTION

DATA

For determining Stewart

the average velocity

and crustal

(1968) has shown that T2 - A2 method

I

2000 A2 (km)

Fig. 4. Travel-time curves for reflected phases.

I

3000

structure

in its simplest

t

4000

from reflected

phases,

form gives reasonably

68

good results. The piot of T' - A’ (Fig. 4) shows that the points fall on a straight which can be represented

line.

as:

T’=XA2+-C where K and C are the constants. readily

computed

The average thickness

from the intercept

of the granitic

layer can be

of P, P line on the T-axis. while its slope gives

average velocity in the layer. The line P,P which is the reflection

related

to the top

the mantle also enables us to compute the average wave velocity and the thickness for the crust as a whole. The average thickness and the velocity in the granitic layer

of

were found to be 18.9 km and 5.70 km/set

while that in the crust as a whole were:

38.6 and 6.5 km/see respectively. Considering the various approximations involved in the method. the results of reflectiun studies appear to be in fairly good agreement with those obtained CODA

from refraction

data.

DURATION

The coda duration is defined here as the time when the signal returns to the noise level present before the onset of the P wave on the record. In a homogeneous structure the variation of coda signal duration with the charge of explosions maintains coherency owing to the uniformity of propagation of the energy in the

Fig. 5. Variatkn Karad.

of logarithm

SF 30 and 80 lie towards

of explosive

charge

west of Karad

(Q} versus

logarithm

of thr signal duration

while SP 120 and 165 towards

its east.

{T) at

69

medium.

However,

basement

layers. A sensitive

this relationship

at Karad

at a magnification

is complicated

micro-earthquake

in case of inhomogeneities

seismograph

in the

(MEQ 800) was installed

of 90 dB with the filter setting

of 5-10

Hz. The Coda

was read from the records of four shot points (SP) 30, 80, 120 and 165. Of these the first two shot points, points,

i.e. SP 30, 80 were west of Karad

i.e. SP 120, 165 were on the eastern

side of Karad.

while the other The logarithm

two shot

of the total

duration of coda was plotted against the logarithm of the charge (Q) of explosive from each of the above SPs keeping the distance (A) constant (Fig. 5). A linear relationship is observed between the two parameters as given in the following equations

by the least squares

method:

SP

30

T = 0.0003Q’~s52

(A = 72.3 km)

SP

80

T = 0.0024Q’.5”

(A = 32.4 km)

SP 120

T = 2.769Q”.48’

(A=

SP 165

T = 5.000Q”.428

(A = 53.6 km)

1.7km)

DISCUSSION

The results obtained from the study in the Koyna region have been compared with those of others (Table 4) in peninsular India and in the Scandinavian region. It is interesting to note a remarkably good agreement with the Scandinavian region so far as layer thicknesses are concerned, while there are some differences in some of the wave velocities (Bath, 1979). It may be seen that there is a general agreement in the crustal structure and the wave velocities as reported from explosion data in peninsular India. Closer examination of Table 4, however, suggests that the value of the Ss velocity in the Koyna region is relatively lower than that in the other parts of the peninsular India (Chaudhury et al., 1984; Srivastava et al., 1983). As mentioned earlier, distance

hot springs

occur along the NNW-SSE

of 320 km between

16 ‘40’N

zone of the Konkan

and 19O35’N

through

which

foothills

over a

the two DSS

profiles were undertaken. These are associated with south-facing fault scarps of volcanics overlooking the Tapti-Pune plain. Gubin (1969) has surmised that the springs arise along a fault zone within the basement. The presence of the springs could be a manifestation of volcanic activity implying higher temperatures and pressures within the crust in the region. It is well known that the seismic wave velocities generally remain uneffected by temperatures up to about 300°C. But at higher temperatures, the change in wave velocities are influenced by the metamorphic process taking place in the region. This would imply that the rocks in the region could possess lower rigidity as compared to other parts of the peninsula, which is reflected through crustal velocities in the region. The phenomenon of seismogram coda is due to scattered energy (Aki, 1969) and is extensively used in the determination of magnitudes of local earthquakes. Of late,

of crustal

Sweden

India

peninsular

section of

Indore-Khandwa

India)

(peninsular

profile (DSS)

Kaveli-Udipi

(DSS)

Koyna profile

Region

Comparison

TABLE 4

6.64

7.03

5.82

6.69

6.81

6.61

p*

4.22

6.01

5.97

5.82

%

7.84

8.03

19.0

18.0

22.4

19.0

19.9

16.2

79.0

17.3

8.23

8.18

Hz

H,

P”

Layer thickness (km) __I_---____

m the Indian

P-wave velocities

from other workers

(km/set)

thickness

38.0

37.9

38.6

36.3

total

3.58

3.38

3.63

3.60

3.41

Ss

4.0h

3.69

-

4.01

4.09

s*

4.55

4.77

4.75

4.60

-

-

18.0

17.8

fl,

_

19.0

18.5

H2

Layer thickness (km)

S-wave velocities (km/xc) S,

Shield as well as other shield regions of the world

37.0

36.3

total

commun.

and pera.

Bath (1979)

et al. (1983)

Srivastava

(1984)

et al

study

Chaudhury

present

References

71

attempts

are being made to utilise it to study the attenuation

earth’s

layers as well. Chaplin

et al. (1980) have inferred

lengths

that there is higher attenuation

of coda waves in southeastern

than in New Hampshire. As for the Koyna region, it may be noticed of charge the coda duration

characteristics

from the earthquake

of the coda

New England

from Fig. 5 that for the same amount

is less for the shots from the west of Karad

than that on

the east. In terms of the scattering theory of coda waves, therefore small signal duration west of Karad implies rapid attenuation of seismic waves and lesser scattering as compared to that in the eastern side. Since it is well known that the Koyna earthquake of 1967 was associated with a north-northeasterly oriented fault (Tandon and Chaudhury, 1968, and others), the above observations could be considered as providing confirmation of a fault between SP 30 and Karad. This is further corroborated by the Deep Seismic Sounding experiments from which Kaila et al. (1981) have reported a fault west of Koyna, although the orientation and the dip of the fault were different from that obtained from the fault plane solutions of the main earthquake of December 1967 (Tandon and Chaudhury, 1968). From a study of shallow seismic refraction, gravity and deep electrical resistivity studies carried out by the Geological Survey of India, Kailasam et al. (1969) have found an abrupt fall in the Trap base roughly 400 m between Koyna and Pophali immediately west of the exposed volcanic scarp suggesting the fault. CONCLUSION

This study has brought out the following interesting results: (1) The velocities of P waves in the Koyna region as computed from refraction data are 5.82, 6.61 and 8.23 km/set in granite, basalt and Moho respectively. The corresponding S wave velocities are 3.41, 4.09 and 4.60 km/set. The velocities of S, waves

in the basement

rock are conspicuously

low as compared

with the other

regions of the peninsular India. The velocities in the granite and crust as a whole as determined from reflection data are 5.70 km/set and 6.5 km/set respectively. (2) The total thickness of crust from both P and S waves are found to be 36.3 km. The thickness of granitic and basaltic layers from P waves have been found to be 17.3 and 19.0 km respectively while that from S waves as 17.8 and 18.5 km respectively. (3) The total thickness of the crust as obtained out to be 38.6 km with 18.9 km of granitic layer.

from the reflection

(4) The Coda durations reveal a marked heterogeneity of Karad in east-west direction and a strong attenuation

study comes

in the crust on either side of waves west of Karad.

ACKNOWLEDGEMENT

The authors wish to thank Mr. V.P. Kamble, Director, for successfully conducting the D.S.S field operations during the first field season. Our thanks are also due to

72

the Director General of Meteorology for according and to the National Geophysical Research Institute, special arrangement

to record shot timings

are also grateful Meteorological comments.

to Dr. A.N. Department

Tandon,

permission Hyderabad

exclusively former

for helpful

for our purpose.

Deputy

suggestions

to publish this work for cooperation and

Director

The authors

General,

and to the referee

India

for valuable

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